This invention relates to a marine drive lower unit with thrust bearing rotation control.
Marine drives normally include an outboard positioned drive unit which includes a gear case for mounting a vertical main drive shaft connected at its upper end to a source of power, such as a marine engine. The lower portion of the drive shaft passes through a lower gear case portion which, in turn, is constructed in a manner to provide a generally horizontal torpedo housing. In the system under consideration here, the housing serves to mount a pair of coaxial propeller shafts which are driven by the main drive shaft and which drivingly rotate a pair of contra-rotating marine propellers which are disposed generally aft of the gear case.
Heretofore, and as illustrated in U.S. Pat. No. 4,529,387, the forward thrust load on the outer shaft for one of the propellers has been carried to the central inner shaft for the other propeller via a single thrust bearing disposed aft of the main drive shaft and mounted directly between the propeller shafts. Because these propeller shafts are driven in a contra-rotating manner, the effective speed on the bearing has necessarily been very high, thus adversely affecting bearing life unless a relatively large size and capacity bearing is utilized.
It is an object of the invention to provide a control for the rotation of the bearing device which transfers forward thrust from the outer to inner propeller shaft, and with the control effectively reducing the rotational speed of the bearing device to reduce the required size and extend the life thereof.
In accordance with the various aspects of the invention, a marine drive unit includes a lower gear case forming a torpedo housing. A pair of coaxial propeller shafts are rotateably mounted in the housing and carry a pair of propellers thereon. The propeller shafts are driven by a pair of opposed driving gears suitably connected through a generally vertical main drive shaft to a marine engine and mounted on the horizontal drive axis. A pair of thrust bearings adapted to carry forward thrust loads are respectively disposed adjacent the facing portions of the opposed driving gears, with the pair being separated by a spacer tightly confined therebetween. The spacer is locked against rotation but is freely floatable in an axial direction, and transfers the forward thrust load from one bearing to the other, so that the load is ultimately transferred from the outer propeller shaft to the inner central shaft. The result is to reduce the rotational speed of each thrust bearing, in this instance by half.
The construction is such that the thrust bearings, although each being subject to approximately the full forward thrust load from the outer propeller shaft, can nevertheless be of reduced size and capacity, as compared to the prior known device.